The description of the prior art will be primarily based on the list of references presented at the end of this section. For convenience, the references will be cited with a numerical xx enclosed in a square bracket, [Ref. xx] or simply [xx].
Concerns over the safety of earlier lithium secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as an anode. There are three fundamentally distinct types of carbonaceous anode materials: (a) graphite, (b) amorphous carbon, and (c) graphitized carbon.
The first type of carbonaceous material includes primarily natural graphite and synthetic graphite (or artificial graphite, such as highly oriented pyrolitic graphite, HOPG) that can be intercalated with lithium and the resulting graphite intercalation compound (GIC) may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to the replacement of lithium metal with the GIC, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g.
Carbon anodes can have a long cycle life due to the presence of a protective surface-electrolyte interface layer (SEI), which results from the reaction between lithium and the electrolyte (or between lithium and the anode surface/edge atoms or functional groups) during the first several charge-discharge cycles. The lithium in this reaction comes from some of the lithium ions originally intended for the charge transfer purpose. As the SEI is formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e. they can no longer be the active element for charge transfer. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer. In addition to SEI formation, Qir has been attributed to graphite exfoliation caused by electrolyte/solvent co-intercalation and other side reactions [Refs. 1-4].
The second type of anode carbonaceous material is amorphous carbon, which contains no or very little micro- or nano-crystallites. This type includes the so-called “soft carbon” and “hard carbon.” The soft carbon is a carbon material that can be readily graphitized at a temperature of 2,500° C. or higher. The hard carbon is a carbon material that cannot be graphitized even at a temperature higher than 2,500° C. In actuality, however, the so-called “amorphous carbons” commonly used as anode active materials are typically not purely amorphous, but contain some small amount of micro- or nano-crystallites. A crystallite is composed of a small number of graphene sheets (basal planes) that are stacked and bonded together by weak van der Waals forces. The number of graphene sheets varies between one and several hundreds, giving rise to a c-directional dimension (thickness Lc) of typically 0.34 nm to 100 nm. The length or width (La) of these crystallites is typically between tens of nanometers to microns.
Among this class of carbon materials, soft and hard carbons made by low-temperature pyrolysis (550-1,000° C.) exhibit a reversible capacity of 400-800 mAh/g in the 0-2.5 V range [Refs. 1-3]. Dahn et al. have prepared the so-called house-of-cards carbonaceous material with enhanced capacities approaching 700 mAh/g [Refs. 1,2]. Tarascon's research group obtained enhanced capacities of up to 700 mAh/g by milling graphite, coke, or carbon fibers [Ref. 3]. Dahn et al. explained the origin of the extra capacity with the assumption that in disordered carbon containing some dispersed graphene sheets (referred to as house-of-cards materials), lithium ions are adsorbed on two sides of a single graphene sheet [Refs. 1,2]. It was also proposed that Li readily bonded to a proton-passivated carbon, resulting in a series of edge-oriented Li—C—H bonds. This provides an additional source of Li+ in some disordered carbons [Ref. 5]. Other researchers suggested the formation of Li metal mono-layers on the outer graphene sheets [Ref. 6] of graphite nano-crystallites. The amorphous carbons of Dahn et al. were prepared by pyrolyzing epoxy resins and may be more correctly referred to as polymeric carbons. Polymeric carbon-based anode materials were also studied by Zhang, et al. [Ref. 7] and Liu, et al. [Ref. 8].
The following mechanisms for the extra capacity over the theoretical value of 372 mAh/g have been proposed [Ref. 4]: (i) lithium can occupy nearest neighbor sites; (ii) insertion of lithium species into nano-scaled cavities; (iii) lithium may be adsorbed on both sides of single layer sheets in very disordered carbons containing large fractions of single graphene sheets (like the structure of a house of cards) [Refs. 1,2]; (iv) correlation of H/C ratio with excess capacity led to a suggestion that lithium may be bound somehow in the vicinity of the hydrogen atoms (possible formation of multi-layers of lithium on the external graphene planes of each crystallite in disordered carbons) [Ref. 6]; and (vi) accommodation of lithium in the zigzag and armchair sites [Ref. 4].
Despite exhibiting a high capacity, an amorphous carbon has a low electrical conductivity (high charge transfer resistance) and, hence, resulting in a high polarization or internal power loss. Conventional amorphous carbon-based anode materials also tend to give rise to a high irreversible capacity due to the existence of too many defect sites that irreversibly trap lithium.
The third type of anode carbonaceous material is graphitized carbon, which includes meso-carbon microbeads (MCMBs) and graphitized carbon fibers (or, simply, graphite fibers). MCMBs are usually obtained from a petroleum heavy oil or pitch, coal tar pitch, or polycyclic aromatic hydrocarbon material. When such a precursor pitch material is carbonized by heat treatment at 400° to 500°, micro-crystals called mesophase micro-spheres are formed in a non-crystalline pitch matrix. These mesophase micro-spheres, after being isolated from the pitch matrix (pitch matrix being soluble in selected solvents), are often referred to as meso-carbon microbeads (MCMBs). The MCMBs may be subjected to a further heat treatment at a temperature in the range of 500° C. and 3,000° C. In order to obtain a stably reversible capacity in an anode, commercially available MCMBs are obtained from heat-treating mesophase carbon spheres at a temperature typically above 2,000° C. and more typically above 2,500° C. for an extended period of time. Graphitized carbons have several drawbacks:                (1) Due to such time-consuming and energy-intensive procedures, MCMBs have been extremely expensive. Likewise, the production of all types of graphite fibers (vapor-grown, rayon-based, pitch based, and polyacrylonitrile-based) is also tedious and energy-intensive and the products are very expensive.        (2) The production of MCMBs having a very small diameter, particularly 5 μm or less has been difficult. When the concentration of optically anisotropic small spheres (meso-phase spheres) increases, the small spheres tend to coalesce and precipitate to produce bulk mesophase and separation of small spheres becomes difficult. This is likely the reason why MCMBs with a bead size less than 5 μm are not commercially available. Smaller anode active material particles are essential to high-rate capacity of a lithium ion battery, particularly for power tool or hybrid vehicle power applications.        (3) Furthermore, both MCMBs and graphite fibers give rise to an anode capacity of typically lower than 350 mAh/g and more typically lower than 320 mAh/g.        
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and the like, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions or react with lithium. Among these materials, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a≦5) are of great interest due to their high theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, in the anodes composed of these materials, severe pulverization (fragmentation of the alloy particles) occurs during the charging and discharging cycles due to expansion and contraction of the anode active material induced by the absorption and desorption of the lithium ions. The expansion and contraction, as well as pulverization of active material particles result in loss of contacts between active particles and conductive additives and loss of contacts between the anode active material and its current collector. This degradation phenomenon is illustrated in FIG. 1. These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, three technical approaches have been followed:    (1) reducing the size of the active material particle (presumably for the purpose of reducing the strain energy that can be stored in a particle, which is a driving force for crack formation in the particle); However, a reduced particle size implies a higher surface area available for potentially reacting with the liquid electrolyte.    (2) depositing the electrode active material in a thin film form directly onto a current collector, such as a copper foil; However, such a thin film structure with an extremely small thickness-direction dimension (much smaller than 500 nm) implies that only a small amount of active material can be incorporated in an electrode, providing a low total lithium storage capacity (even though the capacity per unit mass can be large).    (3) using a composite composed of small electrode active particles supported with or protected by a less active or non-active matrix, e.g., carbon-coated Si particles [Refs. 14-18], sol gel graphite-protected Si, metal oxide-coated Si or Sn [Ref. 12], and monomer-coated Sn nano particles [Ref. 13]. Presumably, the protective matrix provides a cushioning effect for particle expansion or shrinkage, as well as prevents the electrolyte from contacting and reacting with the electrode active material. Examples of anode active particles are Si, Sn, and SnO2. However, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction steps, and other undesirable side effects.
For instance, Hung [Ref. 9] disclosed a method of forming a composite anode material. The steps include selecting a carbon material as a constituent part of the composite, chemically treating the selected carbon material to receive nano particles, incorporating nano particles into the chemically treated carbon material, and removing surface nano particles from an outside surface of the carbon material with incorporated nano particles. A material making up the nano particles alloys with lithium. The resulting carbon/nanoparticle composite anodes did not exhibit any significant increase in capacity, mostly lower than 400 mAh/g, which is not much better than the specific capacity of graphite.
It may be noted that the coating or matrix materials used to protect active particles (such as Si and Sn) are carbon [Ref. 14-18], sol gel graphite [Ref. 19], metal oxide [Ref. 12], monomer [Ref. 13], ceramic [Ref. 10], and lithium oxide [Ref. 11]. These protective materials are all very brittle and/or weak (of low strength). Ideally, the protective material should meet the following requirements: (a) The coating or matrix material should be of high strength and stiffness so that it can help to refrain the electrode active material particles, when lithiated, from expanding to an excessive extent. (b) The protective material should also have high fracture toughness or high resistance to crack formation to avoid disintegration during repeated cycling. (c) The protective material must be inert (inactive) with respect to the electrolyte, but be a good lithium ion conductor. (d) The protective material must not provide any significant amount of defect sites that irreversibly trap lithium ions. The prior art protective materials [e.g., Ref. 10-19] all fall short of these requirements. Hence, it was not surprising to observe that, even with high-capacity Si as an anode active material, the resulting anode typically shows a reversible specific capacity much lower than 1,000 mAh/g (based on per gram of the composite material). Furthermore, in most cases, the electrode was not operated beyond 50 cycles, mostly fewer than 40 cycles.
Further attempts to improve the capacity and cycling stability of a lithium ion battery involved the formation of more complex composite structures. For instance, Si particles were first coated with a shell of SiOx and the resulting core-shell structures were then dispersed in a carbon matrix [Ref. 20,21]. Some improvements have been achieved with this approach, but at the expense of material processing ease. In one case [Ref. 20], the reversible capacity is still low (<800 mAh/g) even though the electrode survives 200 cycles. In the other case [Ref. 21] where a slow and expensive CVD process was used to prepare the complex structure, a specific capacity of 1,500 mAh/g was achieved, but only up to 50 cycles.
Complex composite particles of particular interest are a mixture of separate Si and graphite particles dispersed in a carbon matrix prepared by J. Yang, et al. [Ref. 22-24] and by Mao, et al. [Ref. 27], carbon matrix containing complex nano Si (protected by oxide) and graphite particles dispersed therein [Ref. 25], and carbon-coated Si particles distributed on a surface of graphite particles [Ref. 26]. Again, these complex composite particles led to a specific capacity lower than 800 mAh/g (for up to 30-40 cycles only) [Ref. 22-24], lower than 600 mAh/g (up to 40 cycles) [Ref. 25], or lower than 460 mAh/g (up to 100 cycles) [Ref. 26]. These capacity values and cycling stability are not very impressive. It appears that carbon by itself is relatively weak and brittle and the presence of micron-sized graphite particles does not improve the mechanical integrity of carbon since graphite particles are themselves relatively weak. Graphite was used in these cases presumably for the purpose of improving the electrical conductivity of the anode material. Furthermore, polymeric carbon, amorphous carbon, or pre-graphitic carbon may have too many lithium-trapping sites that irreversibly capture lithium during the first few cycles, resulting in excessive irreversibility.
In summary, the prior art has not demonstrated a composite material that has all or most of the properties desired for use as an anode material in a lithium-ion battery. Thus, there is an urgent and continuing need for a new anode for the lithium-ion battery that has a high cycle life, high reversible capacity, low irreversible capacity, small particle sizes (for high-rate capacity), and compatibility with commonly used electrolytes. There is also a need for a method of readily or easily producing such a material in large quantities.